1. Field of the Invention
The invention relates to a semiconductor laser used as, for example, a light source in optical information processing systems.
2. Description of the Related Art
A semiconductor laser used as a light source in information processing systems is required to have characteristics such as long life, stability of threshold current against variation in temperature, and low noise. When a semiconductor laser is used in an information processing apparatus, noise induced by reflected light returning from other elements of the apparatus causes a serious problem. In order to suppress the noise caused by the returning light, the output of the semiconductor laser was generally modulated at a high frequency to reduce the coherence of the laser light. However, a self-sustained pulsation laser, which makes it possible to periodically pulse laser output with vibrating intensity without using an external modulation circuit, is presently of interest due to its low cost.
In order to obtain self-sustained pulsation, it is necessary for a semiconductor laser to include a saturable absorbing layer, in which the absorption loss varies with the intensity of light. The self-sustained pulsation laser emits laser light with vibrating intensity. This periodical vibration of the light intensity is caused by vibrating carrier population. When the laser first lases, the laser emits high intensity laser light having an intensity which is higher than that of the stationary state and which consumes the majority of the carriers; thus, the light intensity in the second lasing is far less than that of the first laser light due to the reduced carrier population below the threshold value. In this case, the vibration of the light intensity relax with time, and the light intensity attenuates to a constant value.
However, if there is a saturable absorbing layer in the laser, the light absorption becomes high while the light pulse is rising, and the light absorption becomes low while the light pulse is falling. Thus, the saturable absorbing layer makes the light intensity approach zero. Therefore, the saturable absorbing layer suppresses the attenuation by the relaxation and realizes the self-sustained pulsation. Conditions of self-sustained pulsation for a semiconductor laser are reported by Ueno and Lang in the Journal of Applied Physics, vol. 58, p. 1682, 1985. This paper indicates that it is necessary for the saturable absorbing layer to have the same order of loss as that of a resonator and the self-sustained pulsation occurs when the carrier life of the saturable layer is shorter than that of the active layer and when the differential gain coefficient of the saturable absorbing layer is larger than that of the active layer.
One of the methods for producing the saturable absorbing layer is disclosed by Kuroe and Kurihara in Japanese Patent Application, First Publication, No. 61-84891. The same method is also reported by Adachi, et al., in Photonics Technology Letters, vol. 7, p.1406, 1995. FIG. 11 shows a schematic representation of a self-pulsation laser and its band diagram, which are reported by Adachi et al. As illustrated, the laser is constructed by laminating layers on the n type GaAs substrate 11 in the order from the bottom, an n-type clad layer 12 made of n-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P, an undoped quantum-well active layer 14, a saturable absorbing layer 19 made of p-type Ga.sub.0.5 In.sub.0.5 P with a thickness of 5 nm, a light guide layer 61 made of p-type (Al.sub.0.45 Ga.sub.0.55).sub.0.5 In.sub.0.5 P, and a p-type clad layer 20 consisting of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P. It is advantageous to prepare the saturable absorbing layer on the active layer in that the composition or the thickness of the saturable absorbing layer can be selected independently so that the properties of the layer can be controlled with greater freedom and that the light coupling parameter of the saturable absorbing layer with the active layer can be determined with good reproducibility during the crystal growth process. In fact, Adachi, et al., realized the self-sustained pulsation by reducing the carrier life by doping a p-type impurity at a concentration of 2.times.10.sup.18 cm.sup.-3 in the saturable absorbing layer.
An increase of the threshold current at higher temperature is presumed to be caused by a leakage of electrons from the active layer to the clad layer. A measure is proposed to construct an Multiple Quantum Barrier (MQB) in which the barrier of a superlattice is effectively increased by the quantum interference.
However, the self-sustained pulsation of that laser disappears at higher temperatures. Adachi et al., reported in "Extended abstracts of the 43rd Spring Meeting, the Japan Society of Applied Physics and Related Societies 26a-C-10" that it is necessary to decrease the band-gap of the saturable absorbing layer to more than 80 meV lower than that of the active layer, if self-oscillation at 80.degree. C. is desired. However, due to the reduced band-gap, the light absorption of the saturable absorbing layer increases and its threshold current at 25.degree. C. increases to more than 100 mA. The high threshold current generates heat, which deteriorates the reliability of the self-sustained pulsation laser. In order to provide a laser having a long service life, the threshold current must be below 50 mA at room temperature.
Disappearance of the self-sustained pulsation at higher temperatures is caused by leakage of electrons from the active layer, by decreasing the carrier life in the active layer, and by decreasing absorption due to electrons flowing into the saturable absorbing layer. In order to maintain the self-sustained pulsation at higher temperatures, it is necessary to confine electrons so as to prevent them from flowing out from the active layer.
The Multiple Quantum Barrier proposed in the reference is effective for increasing the barrier height for electrons at the .GAMMA.-point in the conduction band, but not effective for electrons at the X-point. In a material system of GaInP/AlGaInP used for a visible-light laser, both energies at .GAMMA. and X-points are similar and the energy difference is as small as several 10 meV, so that electrons passing X-point tend to leak. Accordingly, it is generally difficult to suppress electron-leakage by MQB.